Testosterone Implants in Specific Neural Sites Activate Female Sexual Behaviour

Sexual behaviour in most female mammals is regulated by oestrogen, often acting synergistically with progesterone. Moreover, the most important neural site of action for oestradiol is the ventromedial nucleus. In the female musk shrew, Suncus murinus , testosterone (T) activates sexual behaviour. Virgin females first engage in copulatory behaviour many hours in advance of follicular development and ovulation, when plasma oestradiol levels are very low. Testosterone, produced by the ovaries and the adrenal glands, must be aromatized centrally to oestradiol to initiate sexual behaviour. To identify the neural sites of action for T, ovariectomized females received unilateral hormone implants containing testosterone propionate. Hormone pellets were placed in 1 of several brain sites including the medial preoptic area and the dorsalmedial hypothalamus (DMH). Implants in either of these 2 sites, but not in the lateral preoptic area, internal capsule, nor anterior hypothalamus stimulated the induction of sexual behaviour. Hormone implants in the ventrolateral hypothalamus resulted in partially receptive animals. Immunocytochemistry was employed to determine which steroid receptors were present in the 2 behaviourally active sites. The medial preoptic area (MPO) and the dorsal and ventromedial hypothalamus both contain many cells that express oestrogen receptor immunoreactivity. A smaller subset of neurons in these regions are immunoreactive for androgen receptors. In summary, testosterone can act specifically in either the MPO or the DMH to induce female sexual behaviour. Both sites contain cells that express oestrogen and androgen receptors. Thus, testosterone may work via one or both of these steroid receptors to regulate behaviour. The results of these studies, and data collected previously, show that female sexual behaviour in this species is regulated by T and that this steroid can induce full sexual behaviour in 2 specific neural sites. Both of the sites of action identified in this experiment are critical for the expression of sexual behaviour in other species. The MPO has been shown to regulate male sexual behaviour via the aromatization of T to oestradiol in rodents and birds. The DMH is just dorsal to the ventromedial nucleus (VMN) which is essential for the regulation of sexual behaviour in female rodents. Further work is needed to determine if the neural circuitry for sexual behaviour in the musk shrew involves either the MPO or the VMN, if both areas are essential, or perhaps, the 2 regions regulate different aspects of female sexual behaviour.